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Versions: 00 01 02 03 04 05 06 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 RFC 7267

Network Working Group                                 Luca Martini (Ed.)
Internet Draft                                        Cisco Systems Inc.
Expires: September 2014
Intended status: Standards Track                     Matthew Bocci (Ed.)
Updates: 6073                                         Florin Balus (Ed.)
                                                          Alcatel-Lucent

                                                           March 3, 2014


             Dynamic Placement of Multi-Segment Pseudowires


                  draft-ietf-pwe3-dynamic-ms-pw-21.txt

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 3, 2014

Abstract

   RFC5254 describes the service provider requirements for extending the
   reach of pseudowires (PW) across multiple Packet Switched Network
   domains. A Multi-Segment PW is defined as a set of two or more
   contiguous PW segments that behave and function as a single point-
   to-point PW. This document describes extensions to the PW control
   protocol to dynamically place the segments of the multi-segment
   pseudowire among a set of Provider Edge (PE) routers. This document
   also updates RFC6073 as follows: it updates the



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    value of the length field of the PW Switching Point PE Sub-TLV Type
   0x06 to 14.



Table of Contents

    1        Introduction  .........................................   3
    1.1      Scope  ................................................   3
    1.2      Specification of Requirements  ........................   3
    1.3      Terminology  ..........................................   3
    1.4      Architecture Overview  ................................   4
    2        Applicability  ........................................   5
    2.1      Changes to Existing PW Signaling  .....................   5
    3        PW Layer 2 Addressing  ................................   6
    3.1      Attachment Circuit Addressing  ........................   6
    3.2      S-PE Addressing  ......................................   7
    4        Dynamic Placement of MS-PWs  ..........................   7
    4.1      Pseudowire Routing Procedures  ........................   8
    4.1.1    AII PW Routing Table Lookup Aggregation Rules  ........   8
    4.1.2    PW Static Route  ......................................   9
    4.1.3    Dynamic Advertisement with BGP  .......................   9
    4.2      LDP Signaling  ........................................  11
    4.2.1    Multiple Alternative Paths in PW Routing  .............  13
    4.2.2    Active/Passive T-PE Election Procedure  ...............  13
    4.2.3    Detailed Signaling Procedures  ........................  14
    5        Failure Handling Procedures  ..........................  15
    5.1      PSN Failures  .........................................  15
    5.2      S-PE Specific Failures  ...............................  16
    5.3      PW Reachability Changes  ..............................  16
    6        Operations and Maintenance (OAM)  .....................  17
    7        Security Considerations  ..............................  17
    8        IANA Considerations  ..................................  18
    8.1      Corrections  ..........................................  18
    8.2      LDP TLV TYPE NAME SPACE  ..............................  18
    8.3      LDP Status Codes  .....................................  19
    8.4      BGP SAFI  .............................................  19
    9        References  ...........................................  19
    9.1      Normative References  .................................  19
    9.2      Informative References  ...............................  20
   10        Contributors  .........................................  20
   11        Acknowledgements  .....................................  21
   12        Author's Addresses  ...................................  21








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1. Introduction

1.1. Scope

   [RFC5254] describes the service provider requirements for extending
   the reach of pseudowires across multiple Packet Switched Network
   (PSN) domains. This is achieved using a Multi-Segment Pseudowire
   (MS-PW). An MS-PW is defined as a set of two or more contiguous
   pseudowire (PW) segments that behave and function as a single point-
   to-point PW. This architecture is described in [RFC5659].

   The procedures for establishing PWs that extend across a single PSN
   domain are described in [RFC4447], while procedures for setting up
   PWs across multiple PSN domains, or control plane domains are
   described in [RFC6073].

   The purpose of this document is to specify extensions to the
   pseudowire control protocol [RFC4447], and [RFC6073] procedures, to
   enable multi-segment PWs to be dynamically placed. The procedures
   follow the guidelines defined in [RFC5036] and enable the reuse of
   existing TLVs, and procedures defined for Single-Segment Pseudowires
   (SS-PWs) in [RFC4447]. Dynamic placement of point-to-multipoint
   (P2MP) PWs is for further study and outside the scope of this
   document.




1.2. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.


1.3. Terminology

   [RFC5659] provides terminology for multi-segment pseudowires.

   This document defines the following additional terms:

     - Source Terminating Provider Edge (ST-PE). A Terminating Provider
       Edge (T-PE), which assumes the active signaling role and
       initiates the signaling for multi-segment PW.
     - Target Terminating Provider Edge (TT-PE). A Terminating Provider
       Edge (T-PE) that assumes the passive signaling role. It waits and
       responds to the multi-segment PW signaling message in the reverse
       direction.



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     - Forward Direction: ST-PE to TT-PE.
     - Reverse Direction: TT-PE to ST-PE.
     - Pseudowire Routing (PW routing): The dynamic placement of the
       segments that compose an MS-PW, as well as the automatic
       selection of S-PEs.



1.4. Architecture Overview

   The following figure shows the reference model, derived from
   [RFC5659], to support PW emulated services using multi-segment PWs.



          Native  |<------Multi-Segment Pseudowire------>|  Native
          Service |         PSN              PSN         |  Service
           (AC)   |     |<-Tunnel->|     |<-Tunnel->|    |   (AC)
             |    V     V     1    V     V    2     V    V     |
             |    +----+           +-----+          +----+     |
      +----+ |    |TPE1|===========|SPE1 |==========|TPE2|     | +----+
      |    |------|..... PW.Seg't1....X....PW.Seg't3.....|-------|    |
      | CE1| |    |    |           |     |          |    |     | |CE2 |
      |    |------|..... PW.Seg't2....X....PW.Seg't4.....|-------|    |
      +----+ |    |    |===========|     |==========|    |     | +----+
           ^      +----+           +-----+          +----+       ^
           |   Provider Edge 1        ^        Provider Edge 2   |
           |                          |                          |
           |                          |                          |
           |                  PW switching point                 |
           |                                                     |
           |<------------------ Emulated Service --------------->|



                 Figure 1: MS-PW Reference Model


   The PEs that provide services to CE1 and CE2 are Terminating PE1 (T-
   PE1) and Terminating PE2 (T-PE2), respectively.  A PSN tunnel extends
   from T-PE1 to Switching PE1 (S-PE1) across PSN1, and a second PSN
   tunnel extends from S-PE1 to T-PE2 across PSN2.  PWs are used to
   connect the attachment circuits (ACs) attached to PE1 to the
   corresponding ACs attached to T-PE2.

   A PW segment on PSN Tunnel 1 is connected to a PW segment on PSN
   Tunnel 2 at S-PE1 to complete the multi-segment PW (MS-PW) between
   T-PE1 and T-PE2. S-PE1 is therefore the PW switching point and is



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   referred to as the switching provider edge (S-PE). PW Segment 1 and
   PW Segment 3 are segments of the same MS-PW while PW Segment 2 and PW
   Segment 4 are segments of another MS-PW. PW segments of the same MS-
   PW (e.g., PW segment 1 and PW segment 3) MUST be of the same PW type,
   and PSN tunnels (e.g., PSN1 and PSN2) can be of the same or a
   different technology. An S-PE switches an MS-PW from one segment to
   another based on the PW identifiers ( PWid, or Attachment Individual
   Identifier (AII)). How the PW protocol data units (PDUs) are switched
   at the S-PE depends on the PSN tunnel technology: in case of a
   Multiprotocol Label Switching (MPLS) PSN to another MPLS PSN, PW
   switching involves a standard MPLS label swap operation.

   Note that although Figure 1 only shows a single S-PE, a PW may
   transit more than one S-PE along its path. Although RFC5659 [RFC5659]
   describes MS-PWs that span more than one PSN, this document does not
   specify how the LDP PW control protocol [RFC4447] is used in an
   inter-AS environment.


2. Applicability

   This document describes the case where the PSNs carrying the MS-PW
   are only MPLS PSNs using the Generalized Pseudowire Identifier (PWID)
   Forwarding Equivalence Class (FEC) element (also known as FEC129).

   Interactions with an IP PSN using L2TPv3 as described in [RFC6073]
   section 7.4 are for further study.


2.1. Changes to Existing PW Signaling

   The procedures described in this document make use of existing LDP
   TLVs and related PW signaling procedures described in [RFC4447] and
   [RFC6073]. The following optional TLV is also defined:
     - A Bandwidth TLV to address QoS Signaling requirements (see
       Section 6.2.1).

   This document also updates the value of the length field of the PW
   Switching Point PE Sub-TLV Type 0x06 to 14.












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3. PW Layer 2 Addressing

   Single segment pseudowires on an MPLS PSN can use attachment circuit
   identifiers for a PW using FEC 129. In the case of a dynamically
   placed MS-PW, there is a requirement for the attachment circuit
   identifiers to be globally unique, for the purposes of reachability
   and manageability of the PW.  Referencing figure 1 above, individual
   globally unique addresses MUST be allocated to all the ACs and S-PEs
   of an MS-PW.


3.1. Attachment Circuit Addressing

   The attachment circuit addressing is derived from [RFC5003] AII type
   2, shown here:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AII Type=02  |    Length     |        Global ID              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Global ID (contd.)      |        Prefix                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Prefix (contd.)         |        AC ID                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      AC ID                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 2: AII Type 2 TLV Structure


   The fields are defined in [RFC5003], Section 3.2.

   AII type 2 based addressing schemes permit varying levels of AII
   summarization, thus reducing the scaling burden on PW routing. AII
   Type 2 based PW addressing is suitable for point-to-point
   provisioning models where auto-discovery of the address at the Target
   T-PE is not required.  That is, it is known a-priori by provisioning.

   Implementations of the following procedure MUST interpret the AII
   type to determine the meaning of the address format of the AII,
   irrespective of the number of segments in the MS-PW. All segments of
   the PW MUST be signaled with same AII Type.

   A unique combination of Global ID, Prefix, and AC ID parts of the AII
   type 2 are assigned to each AC. In general, the same Global ID and
   Prefix are be assigned for all ACs belonging to the same T-PE. This
   is not a strict requirement, however. A particular T-PE might have



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   more than one Prefix assigned to it, and likewise a fully qualified
   AII with the same Global ID/Prefix but different AC IDs might belong
   to different T-PEs.

   For the purpose of MS-PWs, the AII MUST be globally unique across all
   PSNs that are spanned by the MS-PW.

   The AII for a local attachement circuit of a given T-PE of an MS-PW
   and the AII of the corresponding attachment circuit on a far-end T-PE
   (with respect to the LDP signaling) are known as the Source
   Attachment Individual Identifier (SAII) and Target Attachment
   Individual Identifier (TAII) as per [RFC6074].



3.2. S-PE Addressing

   Each S-PE MUST be assigned an address which uniquely identifies it
   from a pseudowire perspective, in order to populate the Switching
   Point PE (SP-PE) TLV specified in [RFC6073]. For this purpose, at
   least one Attachment Identifier (AI) address of the format similar to
   AII type 2 [RFC5003] composed of the Global ID, and Prefix part,
   only, MUST be assigned to each S-PE.

   If an S-PE is capable of Dynamic MS-PW signaling, but is not assigned
   with an S-PE address, then on receiving a Dynamic MS-PW Label Mapping
   message the S-PE MUST return a Label Release with the
   "LDP_RESOURCES_UNAVAILABLE" ( 0x38)" status code.


4. Dynamic Placement of MS-PWs

   [RFC6073] describes a procedure for concatenating multiple
   pseudowires together. This procedure requires each S-PE to be
   manually configured with the information required for each segment of
   the MS-PW. The procedures in the following sections describe a method
   to extend [RFC6073] by allowing the automatic selection of pre-
   defined S-PEs, and dynamically establishing a MS-PW between two T-
   PEs.












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4.1. Pseudowire Routing Procedures

   The AII type 2 described above contains a Global ID, Prefix, and AC
   ID. The Target Attachment Individual Identifier (TAII) is used by S-
   PEs to determine the next SS-PW destination for LDP signaling.

   Once an S-PE receives a MS-PW Label Mapping message containing a TAII
   with an AII that is not locally present, the S-PE performs a lookup
   in a PW AII routing table. If this lookup results in an IP address
   for the next-hop PE with reachability information for the AII in
   question, then the S-PE will initiate the necessary LDP messaging
   procedure to set-up the next PW segment. If the PW AII routing table
   lookup does not result in a IP address for a next-hop PE, the
   destination AII has become unreachable, and the PW setup MUST fail.
   In this case the next PW segment is considered un-provisioned, and a
   Label Release MUST be returned to the T-PE with a status message of
   "AII Unreachable".

   If the TAII of a MS-PW Label Mapping message received by a PE
   contains the prefix matching a locally-provisioned prefix on that PE,
   but an AC ID that is not provisioned, then the LDP liberal label
   retention procedures apply, and the Label Mapping message is
   retained.

   To allow for dynamic end-to-end signaling of MS-PWs, information MUST
   be present in S-PEs to support the determination of the next PW
   signaling hop.  Such information can be provisioned (equivalent to a
   static route) on each S-PE, or disseminated via regular routing
   protocols (e.g. BGP).


4.1.1. AII PW Routing Table Lookup Aggregation Rules

   All PEs capable of dynamic MS-PW path selection MUST build a PW AII
   routing table to be used for PW next-hop selection.

   The PW addressing scheme (AII type 2 in [RFC5003]) consists of a
   Global ID, a 32 bit prefix and a 32 bit Attachment Circuit ID.

   An aggregation scheme similar to that used for classless IPv4
   addresses can be employed. An (8 bits) length mask is specified as a
   number ranging from 0 to 96 that indicates which Most Significant
   Bits (MSB) are relevant in the address field when performing the PW
   address matching algorithm.







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    0        31 32    63 64    95 (bits)
   +-----------+--------+--------+
   | Global ID | Prefix | AC ID  |
   +-----------+--------+--------+

                 Figure 3: PW Addressing Scheme


   During the signaling phase, the content of the (fully qualified) TAII
   type 2 field from the FEC129 TLV is compared against routes from the
   PW Routing table. Similar with the IPv4 case, the route with the
   longest match is selected, determining the next signaling hop and
   implicitly the next PW Segment to be signaled.


4.1.2. PW Static Route

   For the purpose of determining the next signaling hop for a segment
   of the pseudowire, the PEs MAY be provisioned with fixed route
   entries in the PW next hop routing table. The static PW entries will
   follow all the addressing rules and aggregation rules described in
   the previous sections.  The most common use of PW static provisioned
   routes is this example of the "default" route entry as follows:

   Global ID = 0 Prefix = 0 AC ID = 0 , Prefix Length = 0 Next Signaling
   Hop = {IP Address of next hop S-PE or T-PE}


4.1.3. Dynamic Advertisement with BGP

   Any suitable routing protocol capable of carrying external routing
   information MAY be used to propagate MS-PW path information among S-
   PEs and T-PEs. However, T-PEs and S-PEs MAY choose to use Border
   Gateway Protocol (BGP) [RFC4271] with the Multiprotocol Extensions as
   defined in [RFC4760] to propagate PW address information throughout
   the PSN. PW address information is only propagated by PEs that are
   capable of PW switching. Therefore, the multiprotocol BGP neighbor
   topology MUST coincide with the topology of T-PEs and S-PEs.

   Contrary to layer 2 VPN signaling methods that use BGP [RFC6074] for
   auto discovery, in the case of the dynamically placed MS-PW, the
   source T-PE knows a-priori (by provisioning) the AC ID on the
   terminating T-PE that signaling should use. Hence there is no need to
   advertise a "fully qualified" 96 bit address on a per PW Attachment
   Circuit basis. Only the T-PE Global ID, Prefix, and prefix length
   needs to be advertised as part of well known BGP procedures - see
   [RFC4760].




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   Since PW Endpoints are provisioned in the T-PEs, the ST-PE will use
   this information to obtain the first S-PE hop (i.e., first BGP next
   hop) to where the first PW segment will be established. Any
   subsequent S-PEs will use the same information (i.e. the next BGP
   next-hop(s)) to obtain the next-signaling-hop(s) on the path to the
   TT-PE.

   The PW dynamic path Network Layer Reachability Information (NLRI) is
   advertised in BGP UPDATE messages using the MP_REACH_NLRI and
   MP_UNREACH_NLRI attributes [RFC4760]. The {AFI, SAFI} value pair used
   to identify this NLRI is (AFI=25, SAFI=6 (pending IANA allocation)).
   A route target MAY also be advertised along with the NLRI.

   The Next Hop field of the MP_REACH_NLRI attribute SHALL be
   interpreted as an IPv4 address, whenever the length of the NextHop
   address is 4 octets, and as a IPv6 address, whenever the length of
   the NextHop address is 16 octets.

   The NLRI field in the MP_REACH_NLRI and MP_UNREACH_NLRI is a prefix
   comprising an 8-octet Route Distinguisher, the Global ID, the Prefix,
   and the AC-ID, and encoded as defined in section 4 of [RFC4760].

   This NLRI is structured as follows:

    Bit
    0     7 8             71 72      103 104  135 136   167
    +------+----------------+-----------+--------+--------+
    |Length|  Route Dist    | Global ID | Prefix | AC ID  |
    +------+----------------+-----------+--------+--------+

                Figure 4: NLRI Field Structure



   The Length field is the Prefix length of the Route Distinguisher +
   Global ID + Prefix + AC-ID in bits.

   Except for the default PW route, which is encoded as a 0 length
   Prefix, the minimum value of the length field is 96 bits. Lengths of
   128 bits to 159 bits are invalid as the AC ID field cannot be
   aggregated. The maximum value of the Length field is 160 bits. BGP
   advertisements received with invalid Prefix lengths MUST be rejected
   as having a bad packet format.








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4.2. LDP Signaling

   The LDP signaling procedures are described in [RFC4447] and expanded
   in [RFC6073]. No new LDP signaling components are required for
   setting up a dynamically placed MS-PW. However, some optional
   signaling extensions are described below.

   One of the requirements that MUST be met in order to enable the QoS
   objectives for a PW to be achieved on a segment is that a PSN tunnel
   MUST be selected that can support at least the required class of
   service and that has sufficient bandwidth available.

   Such PSN tunnel selection can be achieved where the next hop for a PW
   segment is explicitly configured at each PE, whether the PE is a T-PE
   or an S-PE in the case of a segmented PW, without dynamic path
   selection (as per RFC6073). In these cases, it is possible to
   explicitly configure the bandwidth required for a PW so that the T-PE
   or S-PE can reserve that bandwidth on the PSN tunnel.

   Where dynamic path selection is used and therefore the next-hop is
   not explicitly configured by the operator at the S-PE, a mechanism is
   required to signal the bandwidth for the PW from the T-PE to the S-
   PEs. This is accomplished by including an optional PW Bandwidth TLV.
   The PW Bandwidth TLV is specified as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|     PW BW TLV  (0x096E)   |          TLV  Length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Forward SENDER_TSPEC                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Reverse SENDER_TSPEC                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 5: PW Bandwidth TLV Structure



   The PW Bandwidth TLV fields are as follows:

     - TLV Length: The length of the value fields in octets. Value = 64

     - Forward SENDER_TSPEC = The SENDER_TSPEC for the forward direction
       of the PW, as defined in [RFC2210] section 3.1.






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     - Reverse SENDER_TSPEC = The SENDER_TSPEC for the reverse direction
       of the PW, as defined in [RFC2210] section 3.1.


   The complete definitions of the content of the SENDER_TSPEC objects
   are found in [RFC2210] section 3.1. The forward SENDER_TSPEC refers
   to the data path in the direction of ST-PE to TT-PE. The reverse
   SENDER_TSPEC refers to the data path in the direction TT-PE to ST-PE.

   In the forward direction, after a next hop selection is determined, a
   T/S-PE SHOULD reference the forward SENDER_TSPEC object to determine
   an appropriate PSN tunnel towards the next signaling hop. If such a
   tunnel exists, the MS-PW signaling procedures are invoked with the
   inclusion of the PW Bandwidth TLV. When the PE searches for a PSN
   tunnel, any tunnel which points to a next hop equivalent to the next
   hop selected will be included in the search (the LDP address TLV is
   used to determine the next hop equivalence)

   When an S/T-PE receives a PW Bandwidth TLV, once the PW next hop is
   selected, the S/T-PE MUST request the appropriate resources from the
   PSN.  The resources described in the reverse SENDER_TSPEC are
   allocated from the PSN toward the originator of the message or
   previous hop. When resources are allocated from the PSN for a
   specific PW, the SHOULD account for the usage of the resources by the
   PW.

   In the case where PSN resources towards the previous hop are not
   available, the following procedure MUST be followed:
        -i. The PSN MAY allocate more QoS resources, e.g. Bandwidth, to
            the PSN tunnel.
       -ii. The S-PE MAY attempt to setup another PSN tunnel to
            accommodate the new PW QoS requirements.
      -iii. If the S-PE cannot get enough resources to setup the segment
            in the MS-PW a Label Release MUST be returned to the
            previous hop with a status message of "Bandwidth resources
            unavailable"

   In the latter case, the T-PE receiving the status message MUST also
   withdraw the corresponding PW Label Mapping for the opposite
   direction if it has already been successfully setup.

   If an ST-PE receives a Label Mapping message the following procedure
   MUST be followed:

   If the ST-PE has already sent a Label Mapping message for this PW
   then the ST-PE MUST check that this Label Mapping message originated
   from the same LDP peer to which the corresponding Label Mapping
   message for this particular PW was sent. If it is the same peer, the



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   PW is established.  If it is a different peer, then the ST-PE MUST
   send a Label Release message, with a status code of "Duplicate AII"
   to the PE that originated the LDP Label Mapping message.

   If the PE has not yet sent a Label Mapping message for this
   particular PW , then it MUST send the Label Mapping message to this
   LDP peer, regardless of what the PW TAII routing lookup result is.


4.2.1. Multiple Alternative Paths in PW Routing

   A next hop selection for a specific PW may find a match with a PW
   route that has multiple next hops associated with it. Multiple next
   hops may be either configured explicitly as static routes or may be
   learned through BGP routing procedures. Implementations at an S-PE or
   T-PE MAY use selection algorithms, such as CRC32 on the FEC TLV, or
   flow-aware transport PW [RFC6391], for load balancing of PWs across
   multiple next-hops. The details of such selection algorithms are
   outside the scope of this document.


4.2.2. Active/Passive T-PE Election Procedure

   When a MS-PW is signaled, each T-PE might independently initiate
   signaling the MS-PW. This could result in a different path being used
   be each direction of the PW. To avoid this situation one T-PE MUST
   initiate PW signaling (i.e. take an active role), while the other T-
   PE waits to receive the LDP Label Mapping message before sending the
   LDP Label Mapping message for the reverse direction of the PW (i.e.
   take a passive role). The Active T-PE (the ST-PE) and the Passive T-
   PE (the TT-PE) MUST be identified before signaling begins for a given
   MS-PW. Both T-PEs MUST use the same method for identifying which is
   Active and which is Passive.

   A T-PE SHOULD determine whether it assumes the active role or the
   passive role using procedures similar to those of [RFC5036] Section
   2.5.2, Bullet 2. The T-PE compares the Source Attachment Individual
   Identifier (SAII) [RFC6074] with the Target Attachment Individual
   Identifier (TAII) [RFC6074] as unsigned integers, and if the SAII >
   TAII, the T-PE assumes the active role. Otherwise it assumes the
   passive role.

   The following procedure for comparing the SAII and TAII as unsigned
   integers SHOULD be used:

    - If the SAII Global ID > TAII Global ID, then the T-PE is active
        - else if the SAII Global ID < TAII Global ID then the T-PE is
   passive



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          - else if the SAII Prefix > TAII Prefix, then the T-PE is
   active
            - else if the SAII Prefix < TAII Prefix, then the T-PE is
   passive
              - else if the SAII AC-ID > TAII AC-ID, then the T-PE is
   active
                - else if the SAII AC-ID < TAII AC-ID, then the T-PE is
   passive
                  - else there is a configuration error



4.2.3. Detailed Signaling Procedures

   On receiving a Label Mapping message, the S-PE MUST inspect the FEC
   TLV. If the receiving node has no local AII matching the TAII for
   that label mapping then the Label Mapping message SHOULD be forwarded
   on to another S-PE or T-PE. The S-PE will check if the FEC is already
   installed for the forward direction:
     - If the FEC is already installed, and the received Label Mapping
       was received from the same LDP peer to which the forward LDP
       Label Mapping was sent, then this Label Mapping represents
       signaling in the reverse direction for this MS-PW segment.
     - If the FEC is already installed, and the received Label Mapping
       was received from a different LDP peer to which the forward LDP
       Label Mapping was sent, then the received Label Mapping MUST be
       released with the status code of "PW_LOOP_DETECTED".
     - If the FEC is not already installed, then this represents
       signaling in the forward direction.

   The following procedures are then executed, depending on whether the
   Label Mapping was determined to be for the forward or the reverse
   direction of the MS-PW.

   For the forward direction:
        -i. Determine the next hop S-PE or T-PE according to the
            procedures above. If next-hop reachability is not found in
            the S-PE's PW AII routing table then a Label Release MUST be
            sent with status code "AII_UNREACHABLE". If the next-hop S-
            PE or T-PE is found and is the same LDP Peer that sent the
            Label Mapping message then a Label Release MUST be returned
            with the status code "PW_LOOP_DETECTED". If the SAII in the
            received Label Mapping is local to the S-PE then a Label
            Release MUST be returned with status code
            "PW_LOOP_DETECTED".






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       -ii. Check that a PSN tunnel exists to the next hop S-PE or T-PE.
            If no tunnel exists to the next hop S-PE or T-PE, the S-PE
            MAY attempt to setup a PSN tunnel.
      -iii. Check that a PSN tunnel exists to the previous hop. If no
            tunnel exists to the previous hop S-PE or T-PE, the S-PE MAY
            attempt to setup a PSN tunnel.
       -iv. If the S-PE cannot get enough PSN resources to setup the
            segment to the next or previous hop S-PE or T-PE, a Label
            Release MUST be returned to the T-PE with a status message
            of "Resources Unavailable".
        -v. If the Label Mapping message contains a Bandwidth TLV,
            allocate the required resources on the PSN tunnels in the
            forward and reverse directions according to the procedures
            above.
       -vi. Allocate a new PW label for the forward direction.
      -vii. Install the FEC for the forward direction.
     -viii. Send the Label Mapping message with the new forward label
            and the FEC to the next hop S-PE/T-PE.

   For the reverse direction:
        -i. Install the FEC received int he Label Mapping message for
            the reverse direction.
       -ii. Determine the next signaling hop by referencing the LDP
            sessions used to setup the PW in the Forward direction.
      -iii. Allocate a new PW label for the next hop in the reverse
            direction.
       -iv. Install the FEC for the next hop in the reverse direction.
        -v. Send the Label Mapping message with a new label and the FEC
            to the next hop S-PE/ST-PE.


5. Failure Handling Procedures

5.1. PSN Failures

   Failures of the PSN tunnel MUST be handled by PSN mechanisms. An
   example of such a PSN mechanism is MPLS fast reroute [RFC4090]. If
   the PSN is unable to re-establish the PSN tunnel, then the S-PE
   SHOULD follow the procedures defined in Section 10 of [RFC6073].












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5.2. S-PE Specific Failures

   For defects in an S-PE, the procedures defined in [RFC6073] SHOULD be
   followed. A T-PE or S-PE may receive an unsolicited Label Release
   message from another S-PE or T-PE with various failure codes such
   "LOOP_DETECTED", "PW_LOOP_DETECTED", "RESOURCE_UNAVAILBALE",
   "BAD_STRICT_HOP", "AII_UNREACHABLE", etc. All these failure codes
   indicate a generic class of PW failures at an S-PE or T-PE.

   If an unsolicited Label Release message with such a failure status
   code is received at T-PE, then it is RECOMMENDED that the T-PE
   attempt to re-establish the PW immediately. However the T-PE MUST
   throttle its PW setup message retry attempts with an exponential
   backoff in situations where PW setup messages are being constantly
   released.  It is also RECOMMENDED that a T-PE detecting such a
   situation take action to notify an operator.

   S-PEs that receive an unsolicited Label Release message with a
   failure status code SHOULD follow the following procedures:

        -i. If the Label Release is received from an S-PE or T-PE in the
            forward or reverse signaling direction then the S-PE MUST
            tear down both segments of the PW. The status code received
            in the Label Release message SHOULD be propagated when
            sending the Label Release for the next-segment.


5.3. PW Reachability Changes

   In general an established MS-PW will not be affected by next-hop
   changes in AII reachability information.

   If there is a change in next-hop of the AII reachability information
   in the forward direction, the T-PE MAY elect to tear down the MS-PW
   by sending a label withdraw message to downstream S-PE or T-PE. The
   teardown MUST be also accompanied by a unsolicited Label Release
   message, and will be followed by and attempt to re-establish of the
   MS-PW by T-PE.

   If there is a change in the AII reachability information in the
   forward direction at S-PE, the S-PE MAY elect to tear down the MS-PW
   in both directions. A label withdrawal is sent on each direction
   followed by a unsolicited Label Release. The unsolicited Label
   Releases MUST be accompanied by the Status code "AII_UNREACHABLE".
   This procedure is OPTIONAL.  Note that this procedure is likely to be
   disruptive to the emulated service. PW Redundancy [RFC6718] MAY be
   used to maintain the connectivity used by the emulated service in the
   case of a failure of the PSN or S-PE.



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   A change in AII reachability information in the reverse direction has
   no effect on an MS-PW.


6. Operations and Maintenance (OAM)

   The OAM procedures defined in [RFC6073] may be used also for
   dynamically placed MS-PWs.  A PW switching point PE TLV is used
   [RFC6073] to record the switching points that the PW traverses.

   In the case of a MS-PW where the PW Endpoints are identified though
   using a globally unique, FEC 129-based AII addresses, there is no
   pseudowire identifier (PWID) defined on a per-segment basis. Each
   individual PW segment is identified by the address of the adjacent
   S-PE(s) in conjunction with the SAII and TAII.

   In this case, the following TLV type (0x06) MUST be used in place of
   type 0x01 in the PW switching point PE TLV:

   Type      Length    Description
   0x06        14      L2 PW address of PW Switching Point


   The above sub-TLV MUST be included in the Switching Point PE TLV once
   per individual PW Switching Point following the same rules and
   procedures as described in [RFC6073]. A more detailed description of
   this sub-TLV is also given in setion 7.4.1 of [RFC6073]. However, the
   length value MUST be set to 14 (RFC6073 states that the length value
   is 12, but this does not correctly represent the actual length of the
   TLV).


7. Security Considerations

   This document specifies extensions to the protocols already defined
   in [RFC4447], and [RFC6073]. The extensions defined in this document
   do not affect the security considerations for those protocols, but
   [RFC4447] and [RFC6073] do impose a set of security considerations
   that are applicable to the protocol extensions specified in this
   document.

   It should be noted that the dynamic path selection mechanisms
   specified in this document enable the network to automatically select
   the S-PEs that are used to forward packets on the MS-PW. Appropriate
   tools, such as the VCCV Trace mechanisms specified in [RFC6073], can
   be used by an operator of the network to verify the path taken by the
   MS-PW and satisfy themselves that it does not represent an additional
   security risk.



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   Note that the PW control protocol may be used to establish and
   maintain an MS-PW across administrative boundaries. Section 13 of
   [RFC6073] specifies security considerations applicable to LDP used in
   this manner, including considerations on establishing the integrity
   of, and authenticating, LDP control messages. This considerations
   also apply to the protocol extensions specified in this document.

   Note that the protocols for dynamically distributing AII reachability
   information may have their own security considerations. However those
   protocols specifications are outside the scope of this document.



8. IANA Considerations

8.1. Corrections

   IANA is requested to correct a minor error in the registry
   "Pseudowire Switching Point PE sub-TLV Type". The entry 0x06 "L2 PW
   address of the PW Switching Point" should have Length 14 and the
   reference changed to [RFC6073] and [RFC-to-be] as follows:

    Type   Length          Description                     Reference --
   -----+---
   ---+------------------------------------+-------------------------
    0x06     14     L2 PW Address of PW Switching Point  [RFC6073] and
   [RFC-to-be]


8.2. LDP TLV TYPE NAME SPACE

   This document defines one new LDP TLV types. IANA already maintains a
   registry for LDP TLV types called "Type, Length,  and Value (TLV)
   Type Name Space" within the "Label Distribution Protocol (LDP)
   Parameters" as defined by RFC5036. IANA is requested to assign on
   permanent basis the value (0x096E) that has been assigned to this
   document by early allocation (TEMPORARY - Expires 2008-11-21).:

   Value   Description     Reference       Notes/Registration Date
   ------+----------------+---------------+-----------------------
   0x096E  Bandwidth TLV   This document










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8.3. LDP Status Codes

   This document defines three new LDP status codes. IANA maintains a
   registry of these called the "STATUS CODE NAME SPACE" in the "Label
   Distribution Protocol (LDP) Parameters" as defined by RFC5036. The
   IANA is requested to assign on permanent basis the values that has
   been assigned to this document by early allocation
    (TEMPORARY - Expires 2008-11-21):


   Range/Value     E     Description                       Reference
   ------------- -----   ----------------------            ---------
    0x00000037     0     Bandwidth resources unavailable   This document
    0x00000038     0     Resources Unavailable             This document
    0x00000039     0     AII Unreachable                   This document


8.4. BGP SAFI

   IANA needs to allocate a new BGP SAFI for "Network Layer Reachability
   Information used for Dynamic Placement of Multi-Segment Pseudowires"
   from the IANA "Subsequence Address Family Identifiers (SAFI)"
   registry. The IANA is requested to assign on permanent basis the
   values that has been assigned to this document by early allocation
   (TEMPORARY - Expires 2008-11-21)::

   Value    Description                                     Reference
   -----    -----------                                     ---------
   6        Network Layer Reachability Information used This document
            for Dynamic Placement of Multi-Segment
            Pseudowires



9. References

9.1. Normative References

   [RFC6073] Martini et.al. "Segmented Pseudowire", RFC6073,
        January 2011

   [RFC2210] Wroclawski, J. "The Use of RSVP with IETF Integrated
        Services", RFC 2210, September 1997

   [RFC5036] Andersson, Minei, Thomas. "LDP Specification"
        RFC5036, October 2007





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   [RFC4447] "Pseudowire Setup and Maintenance Using the Label
        Distribution Protocol (LDP)", Martini L.,et al, RFC 4447,
        June 2005.

   [RFC5003] "Attachment Individual Identifier (AII) Types for
        Aggregation", Metz, et al, RFC5003, September 2007


9.2. Informative References

   [RFC5254] Martini et al, "Requirements for Multi-Segment Pseudowire
        Emulation Edge-to-Edge (PWE3)",
        RFC5254, Bitar, Martini, Bocci, October 2008

   [RFC5659] Bocci at al, "An Architecture for Multi-Segment Pseudo Wire
        Emulation Edge-to-Edge", RFC5659,October  2009.

   [RFC4760] Bates, T., Rekhter, Y., Chandra, R. and D. Katz,
        "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.

   [RFC6074] E. Rosen, W. Luo, B. Davie, V. Radoaca,
        "Provisioning, Autodiscovery, and Signaling in L2VPNs",
        RFC6074, January 2011

   [RFC4271] Rekhter, Y., et al, "A Border Gateway Protocol 4 (BGP-4)",
        RFC4271, January 2006

   [RFC6391] Bryant, S., et al, "Flow-Aware Transport of Pseudowires
        over an MPLS Packet Switched Network", RFC6391, November 2011

   [RFC4090] Pan, P., et al, "Fast Reroute Extensions to RSVP-TE for LSP
        Tunnels", RFC4090, May 2005

   [RFC6718] Muley, P., et al, "Pseudowire Redundancy", RFC6718, August
        2012


10. Contributors

   The editors gratefully acknowledge the following additional co-
   authors:  Mustapha Aissaoui, Nabil Bitar, Mike Loomis, David McDysan,
   Chris Metz, Andy Malis, Jason Rusmeisel, Himanshu Shah, Jeff
   Sugimoto.








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11. Acknowledgements

   The editors also gratefully acknowledge the input of the following
   people:  Mike Duckett, Paul Doolan, Prayson Pate, Ping Pan, Vasile
   Radoaca, Yeongil Seo, Yetik Serbest, Yuichiro Wada.



12. Author's Addresses


   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112
   e-mail: lmartini@cisco.com


   Matthew Bocci
   Alcatel-Lucent,
   Voyager Place
   Shoppenhangers Road
   Maidenhead
   Berks, UK
   e-mail: matthew.bocci@alcatel-lucent.com


   Florin Balus
   Alcatel-Lucent
   701 E. Middlefield Rd.
   Mountain View, CA 94043
   e-mail: florin.balus@alcatel-lucent.com


   Nabil Bitar
   Verizon
   40 Sylvan Road
   Waltham, MA 02145
   e-mail: nabil.bitar@verizon.com


   Himanshu Shah
   Ciena Corp
   35 Nagog Park,
   Acton, MA 01720
   e-mail: hshah@ciena.com





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   Mustapha Aissaoui
   Alcatel-Lucent
   600 March Road
   Kanata
   ON, Canada
   e-mail: mustapha.aissaoui@alcatel-lucent.com


   Jason Rusmisel
   Alcatel-Lucent
   600 March Road
   Kanata
   ON, Canada
   e-mail: Jason.rusmisel@alcatel-lucent.com


   Yetik Serbest
   AT&T Labs
   9505 Arboretum Blvd.
   Austin, TX 78759
   e-mail: yetik.serbest@labs.att.com


   Andrew G. Malis
   Huawei
   2330 Central Expressway
   Santa Clara CA 95050
   e-mail: agmalis@gmail.com


   Chris Metz
   Cisco Systems, Inc.
   3700 Cisco Way
   San Jose, Ca. 95134
   e-mail: chmetz@cisco.com


   David McDysan
   Verizon
   22001 Loudoun County Pkwy
   Ashburn, VA, USA 20147
   e-mail: dave.mcdysan@verizon.com








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   Jeff Sugimoto
   Alcatel-Lucent
   701 E. Middlefield Rd.
   Mountain View, CA 94043
   e-mail: jeffery.sugimoto@alcatel-lucent.com


   Mike Duckett
   ATT
   Lindbergh Center D481
   575 Morosgo Dr
   Atlanta, GA  30324
   e-mail: md9308@att.com


   Mike Loomis
   Alcatel-Lucent
   701 E. Middlefield Rd.
   Mountain View, CA 94043
   e-mail: mike.loomis@alcatel-lucent.com


   Paul Doolan
   Coriant GmbH & Co. KG
   St Martin Str. 76
   81541 Munich
   e-mail: paul.doolan@coriant.com


   Ping Pan
   Infinera
   e-mail: ppan@infinera.com


   Prayson Pate
   Overture Networks, Inc.
   507 Airport Blvd, Suite 111
   Morrisville, NC, USA 27560
   e-mail: prayson.pate@overturenetworks.com











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   Vasile Radoaca
   Alcatel-Lucent
   388 NINGQIAO RD
   PUDONG JINQIAO
   SHANGHAI 201206
   CHINA
   email: vasile.radoaca@alcatel-lucent.com


   Yuichiro Wada
   NTT
   3-9-11 Midori-cho
   Musashino-shi
   Tokyo  180-8585
   Japan
   e-mail: wada.yuichiro@lab.ntt.co.jp


   Yeong-il Seo
   Korea Telecom Corp.
   463-1 Jeonmin-dong, Yusung-gu
   Daejeon, Korea
   e-mail: yohan.seo@kt.com



Copyright Notice

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   Provisions Relating to IETF Documents
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   publication of this document. Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008. The person(s) controlling the copyright in some of this
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling



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   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
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   Expiration Date: September 2014












































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